We will test the hypothesis that some factor(s) interact(s) with menopause to predispose some but not other postmenopausal women to osteoporosis by comparing osteoporotic and control postmenopausal women before and after eliminating the confounding variable of estrogen deficiency by treatment. Complete metabolic studies will be made in 16 normal and 16 osteoporotic postmenopausal women before and after one year of treatment with estrogen or placebo.